[Haskell-cafe] How to generate dependend values with QuickCheck

[Jürgen Nicklisch-Franken]

Thank you all, (you are responding so quick that I consider to stop
thinking myself in the future and just ask).
Jürgen

Am Donnerstag, den 26.08.2010, 00:02 -0400 schrieb Edward Z. Yang:
> Excerpts from Jürgen Nicklisch-Franken's message of Wed Aug 25 23:32:51 -0400 2010:
> > instance Arbitrary QCExample where
> >     arbitrary =
> >         let i1 = arbitrary
> >             i2 = fmap (* 2) i1
> >         in liftM2 QCExample i1 i2
> 
> What's happening here is that you are "evaluating" the random value generator
> too late in the game: when you set i1 = arbitrary, no random value has been
> generated yet, and i2 is thus another generator which has no relation to
> what i1 might produce.  You could instead do this:
> 
>     arbitrary = do
>         i1 <- arbitrary
>         let i2 = i1 * 2
>         return (QCExample i1 i2)
> 
> Cheers,
> Edward